Over the years skid steer loaders have been known as agile, compact vehicles with a high degree of maneuverability and a wide range of applications in the agricultural, industrial and construction fields. These vehicles usually include an engine, a boom assembly and an operators compartment mounted on a frame supported by four ground supporting wheels. Coupled to the engine are a main drive system and a lift system for the boom assembly. The vehicle is maneuvered by driving the wheels on one side at a different speed and/or in a different direction from those on the other side resulting in a turning motion, the severity of which is determined by the relative speeds.
Typically the engine, which is rear mounted for counterbalancing effect, drives a pair of hydrostatic pumps coupled to left and right mounted hydrostatic motors. Wheels on the left and right sides of the vehicle are driven by the left and right mounted motors through gears, chains and sprockets. Motion is usually controlled by an operator seated within the operators compartment by actuating a pair of control levers which are linked to the pumps. The extent to which each lever is moved in a forward direction from a neutral position controls the amount of fluid supplied in a forward direction to its respective motor, and therefore the speed at which the wheels on that side of the vehicle will rotate. Similarly, the extent to which a lever is moved in the reverse direction from the neutral position will control the speed at which the associated wheels rotate in the reverse direction.
As mentioned above, skid steer loaders include a boom assembly. In one common arrangement the boom assembly comprises a pair of lift arms pivotally mounted directly to the main frame, or a support frame extending upwardly from the main frame as shown in U.S. Pat. No. 3,903,978, issued Sep. 9, 1975 in the name of Peter B. Kraus. Material handling attachments, such as the bucket 18 shown in this patent, are usually mounted on the front of the lift arms. U.S. Pat. No. 3,961,131, issued Jun. 15, 1976 in the name of Donald J. Dimmer, and U.S. Pat. No. 4,892,155, issued Jan. 9. 1990 in the name of Richard B. Wanamaker, also are representative of skid steer loader type vehicles having lift arms pivotally affixed to the main frame.
Another well known arrangement found in vehicles of this type comprises a pair of lift arms coupled to the frame by means of a linkage assembly, as best illustrated by U.S. Pat. No. 3,215,292, issued Nov. 11, 1965 in the name of Lawrence M. Halls. This linkage arrangement enhances the path of the bucket by moving it outwardly from vertical as it rises. Similar linkage systems on skid steer loaders type vehicles are disclosed in U.S. Pat. No. 3,995,761, issued Dec. 7, 1976 in the name of Joseph C. Hurlburt and U.S. Pat. No. 4,355,946, issued Oct. 26, 1982 in the name of Lloyd A. Wykhuis, et al.
A separate hydraulic system is usually used in skid steer loaders to power the boom assembly via hydraulic lift cylinders coupled to the lift arms. This same system can also be used to actuate one or two tilt cylinders which pivot the attachment relative to the lift arms, which is commonly referred to as dumping or curling the attachment. Typically, a pair of foot pedals in the front of the operator compartment control the flow of hydraulic fluid from a hydraulic pump to the lift and tilt cylinders.
In addition to material handling buckets, various other attachments such as snow blowers, trenchers, tree spades and augers which include their own hydraulic motors and/or cylinders are commonly mounted to the boom assembly. An auxiliary hydraulic system is used to control the flow of hydraulic fluid between the pump and the hydraulic motor of the front mounted attachment. It is common in prior art systems for the flow of hydraulic fluid to the motor to be controlled by an auxiliary spool valve through actuation of a handle on one of the control levers. The handle is normally biased to a neutral position. Pushing the handle in one direction strokes the auxiliary valve in a first direction, thereby causing hydraulic fluid to flow to the front mounted attachment in a first direction. Pushing the handle in the opposite direction strokes the auxiliary valve so as to supply fluid in a reverse direction.
Of the wide range of attachments used in conjunction with skid steer loaders the most common is a material handling bucket mounted to pivot relative to the lift arms. The bucket is operational over its full lift path, e.g., it is generally loaded while resting on or in the vicinity of the ground, it is then pivoted upwardly and lifted to an intermediate position for transport to a remote unloading area, and then it is unloaded by pivoting it downwardly to discharge its contents. When unloading takes place at an elevated bucket position relative to the loading position such as into the back of a dump truck, over the side of a manure spreader, etc., it is an advantage to have the bucket reach forward from its initial position, as it is being lifted. This is accomplished by the linkage system featured in the Halls patent mentioned above. This is a known advantage over systems wherein the lift arms have a fixed pivot causing the buckets to lift through an arcuate path. Because the pivot is above the bucket, the path initially moves toward the vehicle being loaded as it rises and then as it continues above the vertical height of the pivot, the path arcs back in the opposite direction, which requires the operator to simultaneously coordinate curl of the bucket with forward movement of the vehicle to place the bucket in its proper unload position.
Another common usage of skid steer loaders involves the necessity of maintaining the loaded material and thus the bucket in a stable condition as it is initially being lifted from the ground to a transport position. In prior art systems the bucket path is forwardly inclined initially, which again requires the operator to simultaneously coordinate controls, i.e., in this instance the bucket curl and lift controls are involved to avoid spillage or premature dumping of material being transported in the bucket. For example, see U.S. Pat. No. 4,162,873, issued Jul. 31, 1979 in the name of Raymond E. Smith, Jr., and U.S. Pat. No. 4,844,685, issued Jul. 4, 1989 in the name of Thomas M. Sagaser.
In all known prior art apparatus of which applicants are aware there is no system that provides for both an optimum initial vertical path as well as a maximum reach position below maximum height while still substantially maintaining such reach as the bucket continues to its maximum height position.